The present invention relates to an active filter circuit widely used for analog signal processing and, more particularly, to an active filter circuit applied to a MOS (metal oxide semiconductor) or MIS (metal insulator semiconductor) transistor IC circuit.
A variable GM amplifier constituted of current-output type differential amplifiers has been used as an active filter included in an IC capable of obtaining a desired frequency response. Various types of filter can be formed by applying the above variable GM amplifier to an active filter circuit.
FIG. 1 is a circuit diagram showing an arrangement of a trap filter using prior art GM amplifiers. The trap filter includes variable GM amplifiers 21 and 22. A capacitor C31 is inserted between a ground and a node of an output terminal of the GM amplifier 21 and a noninverting input terminal of the GM amplifier 22. A capacitor C32 is inserted between a noninverting input terminal of the GM amplifier 21 and an output terminal of the GM amplifier 22. The output terminal of the GM amplifier 22 is connected to an inverting input terminal of the GM amplifier 21 through a buffer 23 and to that of the GM amplifier 22 through a buffer circuit 24 for controlling selectivity Q of the filter. The GM amplifiers 21 and 22 are supplied with a control signal in order to obtain a desired frequency response. It is an input signal X of the filter that is supplied to the noninverting input terminal of the GM amplifier 21, while it is an output signal Y of the filter that is output from the GM amplifier 22 through the buffer 23.
In the above arrangement, a transfer function of the output signal Y relative to the input signal X is expressed by the following equation: ##EQU1##
where s is j.omega., gm1 and gm2 are transconductances of the GM amplifiers 21 and 22, and C.sub.31 and C.sub.32 are capacitances of the capacitors C31 and C32.
As is apparent from the above equation (1-1), a desired frequency response can be obtained by controlling the transconductances gm1 and gm2 of the variable GM amplifiers 21 and 22. For simplification of description, it is assumed that the selectivity Q of the filter is fixed.
FIG. 2 is a circuit diagram of a specific circuit arrangement generally used in the variable GM amplifiers shown in FIG. 1. In this arrangement, the bases of differential pair transistors Q1 and Q2 are a noninverting input and an inverting input, respectively. Transistors Q3 and Q4 whose bases are connected to a voltage source VB, are voltage-to-current converters, the collectors thereof are connected to a power supply Vcc, and the emitters thereof are connected to the collectors of the transistors Q1 and Q2, respectively. The emitters of the transistors Q1 and Q2 are connected to each other via a resistor R for determining a current conversion coefficient, and grounded through their respective constant-current sources of constant current I1.
The bases of transistors Q5 and Q6 of an output control system are connected to their respective collectors of the transistors Q2 and Q1. Both the emitters of the transistors Q5 and Q6 are grounded through a constant-current source of constant current I2. The collectors of the transistors Q5 and Q6 output a differential current.
The circuit shown in FIG. 2 is a basic Gilbert circuit, and a transfer function from input V.sub.i to output current I.sub.o (=I5-I6) is given by the following equation: ##EQU2##
where I.sub.1 and I.sub.2 are values of constant currents I1 and I2 in FIG. 2.
As is evident from the above equation (2-1), the transconductance gm of the circuit illustrated in FIG. 2 is expressed by the following equation: ##EQU3##
As is evident from the equation (3-1), the transconductance gm is controlled by a ratio of I.sub.1 to I.sub.2. It is understood that the frequency response of the filter shown in FIG. 1 is variable if the constant currents I1 and I2 are control signals in the filter circuit while I1 is a fixed current and I2 is a variable current.
The above prior art variable GM amplifiers are so constituted that they compress and expand an input signal and transfer it, making use of diode characteristics of a bipolar transistor. Thus, the prior art amplifiers have the following problem.
Let us consider noise performance first. In the circuit arrangement shown in FIG. 2, an input signal is compressed by a differential circuit constituted of the pairs of transistors Q1 and Q2 and transistors Q3 and Q4 and then expanded by the pair of transistors Q5 and Q6.
The noise dominant over the above circuit is a shot noise of the pair of transistors Q3 and Q4 and that of transistors Q5 and Q6. The shot noise is thus added to the compressed input signal to thereby deteriorate the noise performance. Moreover, noise is caused to such an extent that a thermal noise of in-base resistance (rbb') of each transistor is not negligible.
The following two methods are generally adopted in order to improve the noise performance described above:
(1) The currents I1 and I2 are increased to reduce an input conversion noise. In other words, the I/O dynamic range is increased to improve the S/N ratio equivalently.
(2) The base area of transistors is increased to reduce the in-base resistance rbb' and improve the noise performance.
It is likely that the above two methods will improve the noise performance to some extent, but it is inevitable that they will increase the current consumption. The increase in current requires a transistor of a certain size. In order to lower the in-base resistance rbb', a larger-sized transistor has to be employed. The device size is increased accordingly.
In a commonly-used filter circuit as described above, the noise performance as well as the frequency response required in accordance with its uses is considered to be important. Moreover, the filter circuit is required very strongly to decrease in power consumption and increase in degree of integration in accordance with recent multifunction and high performance of an IC.
Using a prior art variable GM amplifier makes it difficult to improve in filter performance including noise performance, reduce in power consumption, and increase in degree of integration at the same time.
Next, let us consider that a differential amplifier is constituted of MOS or MIS transistors. Using MOS or MIS transistors, both low power consumption and high degree of integration can be expected.
FIG. 3 is a circuit diagram of a generally-used differential amplifier constituted of MOS transistors. In this amplifier, the sources of N-channel MOS transistors M41 and M42 whose gates are supplied with a differential input signal, are grounded through their common constant-current source I.sub.o. The sources of P-channel MOS transistors M43 and M44 are connected in common to the power supply, and the drains thereof are connected to their respective drains of the MOS transistors M41 and M42. The gates of the MOS transistors M43 and M44 are connected in common to the drain of the MOS transistor M43 to constitute a current mirror circuit. A current Iout is output from a drain node of the MOS transistors M42 and M44.
In FIG. 3, currents i.sub.11 and i.sub.12 are given by the following equations in view of the characteristics of MOS transistors if V.sub.1, V.sub.2 and V.sub.m are voltages and g is conductance: EQU i.sub.11 =g(V.sub.1 -V.sub.m -V.sub.th).sup.2 EQU i.sub.12 =g(V.sub.2 -V.sub.m -V.sub.th).sup.2
Since i.sub.11 +i.sub.12 =I.sub.o (constant current), the output current is expressed by the following equation when .DELTA.V=V.sub.1 -V.sub.2 and i.sub.out =i.sub.11 -i.sub.12 : ##EQU4##
As is seen from the equation (4-1), the output current i.sub.out is not made linear relative to an input .DELTA.V but caused to have a second-order distortion. Consequently, even though a filter circuit includes a differential amplifier constituted of MOS or MIS transistors, a distortion easily occurs and thus any measures have to be taken.